1. Field of the Invention
The present invention generally relates to a film forming method for a dielectric film, and in particular relates to a film forming method for a dielectric film having a low dielectric constant.
2. Background Art
In a semiconductor device that has been recently developed to have a relatively fine structure, the so-called multi-layer wiring structure is used for electrically connecting a great number of semiconductor elements formed on a substrate. In such a multi-layer wiring structure, interlayer insulating films, in each of which wiring patterns are embedded, are laminated in large numbers one on another. One layer of the wiring patterns is connected with another adjacent layer of the wiring patterns or diffusion region formed in the substrate, relative to each other, via a contact hole or holes formed through each interlayer insulating film.
In each interlayer insulating film of such a finely structured semiconductor device, complicated wiring patterns are formed adjacent to one another. Therefore, wiring delay of a signal due to parasitic capacitance in such an interlayer insulating film has been seriously problematic.
Therefore, especially, in the recent semiconductor device having a hyperfine structure on the order of the so-called sub-microns or sub-quarter-microns, a fluorinated silicon oxide film (i.e., SiOF film) having a relative dielectric constant of 3 to 3.5 is used as each interlayer insulating film for constituting the multi-layer wiring structure, in place of a conventional silicon oxide film (i.e., SiO2 film) having the relative dielectric constant of approximately 4.
Even by the use of such an SiOF film, however, reduction of the relative dielectric constant has been limited. Namely, for the SiOF-based insulating film, it is difficult to achieve a relative dielectric constant of 3.0 or less that is required for a semiconductor device of a generation to be produced under a designing rule of 0.1 μm or less.
As an insulating film having a lower relative dielectric constant, i.e., the so-called low-k insulating film, various materials have been known. However, the interlayer insulating film used in the multi-layer structure is required to have not only such a lower relative dielectric constant but also high mechanical strength and excellent stability against a heating process.
An SiOCH film exhibits sufficient mechanical strength, and also it can achieve a relative dielectric constant of 2.5 or less. Besides, the SiOCH film can be formed by a CVD method that is convenient for a production process for the semiconductor device. Accordingly, the SiOCH film may be a promising material as a low-k interlayer insulating film to be used for an ultrahigh-speed semiconductor device for the next generation.
In WO2005/045916, a technique for forming a porous low-k film is described, wherein the SiOCH film formed by a plasma CVD method using an organic silane gas as a raw material is modified by a hydrogen plasma process, so that CHx groups and/or OH groups in the film can be released therefrom, thereby to reduce a film density thereof.
In the conventional multi-layer wiring structure, such a porous low-k film has been employed in layers located in the vicinity of the substrate of the device, in which fine wirings are formed in a higher density.
Recently, however, there is a demand for providing such a porous low-k film also in upper layers in which global wirings are formed. Generally, each interlayer insulating film, in which the global wiring is formed, has a film thickness of 1 μm or greater.
The interlayer insulating films constituting an upper layer portion of the multi-layer wiring structure are required to support a bonding pad. Therefore, these films should have not only a lower dielectric constant but also a higher modulus of elasticity. However, since the conventional low-k film or porous film has a lower modulus of elasticity, it is difficult to employ such a low-k film or porous film in the upper layer portion of the multi-layer wiring structure.
Besides, the low-k film or porous film, such as the SiOCH film or the like, used as a low-k interlayer insulating film, generally has a lower adhesiveness associated with its lower density. Accordingly, in the case of forming such a film on an etching stopper film, such as an SiC film, SiN film, SiCN film or SiCO film, it is likely to be peeled off from the etching stopper film.